1. Field of the Invention
This invention relates in general to container closures, and more particularly to a container closure having multiple sealing surfaces and a tamper-evident ring.
2. Description of the Related Art
Various tamper-evident closures have been proposed wherein a threaded cap has a skirt portion with internal threads for connecting the cap to the threaded neck of a container. A security ring is typically formed during molding of the cap and frangible connections are formed after molding by cutting or slitting openings into the closure at the juncture of the security ring and skirt portion. The security ring may be provided with a plurality of circumferentially spaced, inwardly and upwardly extending tabs or fingers integrally connected to the ring. The outer free ends of the fingers are adapted to engage beneath a shoulder or ledge on the container neck below the threaded portion. In this manner, the cap is prevented from being removed from the container until twisted a sufficient distance to break the frangible connection and thereby separate the cap from the ring.
Closures of the above-mentioned type are typically manufactured with medium or high density polyethylene materials through die casting or thermoforming. The materials must be sufficiently resilient and flexible to enable extraction of the male die member from the cavity of the closure despite the presence of the inwardly and upwardly extending fingers. Since the closures are anchored to the container mainly by engagement of the fingers with the shoulder or ledge, the required flexibility and resilience of the closure to enable separation from the die may also enable the closure to be relatively easily removed from the container without breaking the frangible connection between the skirt and the security ring by slipping the closure off the container in a direction parallel with the axis of the container neck. This is particularly disadvantageous since there is no clear indication that the container has been tampered with.
In recognition of this problem, UK Patent Application No. GB 2 039 817 proposes die molding a closure out of relatively rigid plastics material, such as polypropylene. However, the die-molding process itself is relatively time consuming and therefore contributes to an increase in the cost of manufacturing the closure. In addition, the formation of frangible bridges requires a further processing step wherein openings between the bridges are cut or slit at the intersection of the security ring and skirt portion of the closure. The extra processing step requires extra equipment, time, and energy and thereby contributes to the overall cost of the closure.
Other closure designs depend on adding a different material on the bottom surface of the closure top wall to maintain sealing against leakage. The addition of any material to the cap requires more steps in manufacturing and assembly, and also contributes to an increase in closure manufacturing costs. Thus, it is desirous to produce a closure that is of single piece construction.
As desirable as the one-piece plastic closure may be, there are a number of problems associated with its use. For example, it is difficult to maintain a seal between the closure and container over an extended period of time under various conditions of packing, storing and handling. Once the closure is firmly engaged on the container, whether by threads or some other means, various parts of the closure are under stress because of forces required to effect a seal. These stresses tend to cause cold flow or creep of the closure material and may cause a breach in the seal between the closure and container. This tendency is augmented when the closure and container are subject to internal pressure from carbonated beverages. A plastic material""s tendency to creep is further affected by elevated temperatures, which may result in a breach of the seal and consequential leakage and/or spoilage of the contents.
Even when the closure is sufficient to retain the beverage in a carbonated state, removal of the closure for dispersing the beverage causes seal degradation to the point where the closure cannot retain its prior sealing ability. Consequently, the beverage will lose much of its carbonation in a relatively short period of time and is often discarded before being completely consumed.
In addition to the above drawbacks, the internal threads in prior art closures are typically symmetrical in cross section. Although this construction may facilitate attachment of the closure to the container, the internal threads may not properly seal against the external threads of the container. Moreover, the molding process for closures with threads of this nature typically requires the core mold to be rotated or unscrewed from the interior of the closure after closure formation. This is a time consuming process and is much less efficient than if the core mold were to be withdrawn from the formed closure without rotation.
According to one embodiment of the invention, a closure is provided for connection to a container. The container typically has a neck portion and an external helical thread formed on an outer surface of the neck portion. The closure comprises an upper wall and a skirt formed integrally with and extending generally downwardly from the upper wall. The skirt includes an inner surface with an internal helical thread formed thereon for engaging the external helical thread of the neck portion to thereby secure the closure to the container. The internal helical thread is of asymmetrical shape in cross section and has a crown spaced from the skirt inner surface, an upper thread surface extending generally upwardly and away from the crown to the skirt inner surface, and a lower thread surface extending generally downwardly and away from the crown to the skirt inner surface. A length of the upper thread surface is greater than a length of the lower thread surface to thereby form the asymmetrical shape. Preferably, the upper thread surface is shaped to sealingly engage a lower surface of the external helical thread to thereby hold the closure on the container against relatively high internal container pressure acting on the closure.
According to a further embodiment of the invention, a closure comprises an upper wall and a skirt formed integrally with and extending generally downwardly from the upper wall. The skirt has an inner surface with an internal helical thread formed thereon for engaging the external helical thread of the neck portion to thereby secure the closure to the container. The internal helical thread comprises a crown spaced from the skirt inner surface, an upper thread surface extending generally upwardly and away from the crown to the skirt inner surface, and a lower thread surface extending generally downwardly and away from the crown to the skirt inner surface. An absolute value of an average slope of the upper thread surface is greater than an absolute value of an average slope of the lower thread surface to thereby form an asymmetrical cross sectional shape.
According to an even further embodiment of the invention, a closure comprises an upper wall, a skirt formed integrally with and extending generally downwardly from the upper wall, and a tamper-evident security ring connected to the skirt. The skirt includes an inner skirt surface with an internal helical thread formed thereon for engaging the external helical thread of the neck portion to thereby secure the closure to the container. The internal helical thread comprises a crown spaced from the skirt inner surface, an upper thread surface extending generally upwardly and away from the crown to the skirt inner surface and a lower thread surface extending generally downwardly and away from the crown to the skirt inner surface. Preferably, the length of the upper thread surface is greater than the length of the lower thread surface to thereby form an asymmetrical cross sectional shape. The upper thread surface is adapted to sealingly engage a lower surface of the external helical thread when the closure is installed on the neck portion. A first annular seal is formed on the inner surface and extends generally radially inwardly toward a central axis of the closure. The first annular seal is adapted to sealingly engage the outer surface of the neck portion. A second annular seal extends generally axially downwardly from the upper wall. The second annular seal is adapted to sealingly engage an inner surface of the neck portion. Preferably, a thickness of the neck portion between the first and second annular seals is greater than the distance between the first and second annular seals before installation of the closure on the container, such that when the closure is installed on the container, the first and second annular seals are biased toward each other with the first annular seal exerting inward radial pressure on the outer surface of the neck portion and the second annular seal exerting outward radial pressure on the inner surface of the neck portion. A third annular seal is located on a lower surface of the upper wall between the first and second annular seals. The third annular seal is adapted to engage an upper edge of the neck that extends between the inner and outer surfaces of the neck portion. The tamper-evident security ring is adapted to contact an annular shoulder of the container neck portion. The security ring comprises an annular wall and a plurality of circumferentially spaced retaining tabs. Each retaining tab has an inner end resiliently connected to the annular wall through an integral hinge and an outer free end. Installation of the closure on the neck portion causes the retaining tabs to elastically deflect toward the annular wall when the outer free ends pass by the shoulder and then to bias against the outer surface of the neck portion below the annular shoulder when the closure is installed on the neck portion.